Sorry, I think I'm wrong. When I see two exercises per muscle group, I always assume it belongs to a classic upper/lower routine. Once every 5 days may be okay. If you keep making progress, stick to it. You can always add a little more volume along the way.

Sure, you could move them to back day and you won't die or anything. But the reason that triceps are paired with chest is because most chest exercises hit the triceps as well. Moving triceps to back day certainly won't be ideal for recovery.